The present invention relates to an apparatus for developing electrostatic latent images carried on a photosensitive surface by a magnetic brush method.
In electrostatic latent image developing apparatuses in photocopiers, electrostatic recording apparatuses and the like, electrostatic latent images are formed on an image-carrying surface made from selenium or zinc oxide photosensitive materials, organo-photoconductive materials, etc., developed with magnetic developers by way of a magnetic brush method, and fixed on papers, or transferred to transfer sheets and then fixed.
As magnetic developers for the magnetic brush method, two-component developers consisting of ferromagnetic carriers and toners are widely used. The magnetic carriers are iron particles, ferrite particles, nickel particles, etc. which may be coated with organic polymers. The toners are fine resin particles containing coloring pigments or dyes dispersed therein. The carrier and toner are selected so that they are charged in opposite polarities by friction when mixed and stirred.
When the development of the electrostatic latent images is carried out using a two-component developer, the concentration of toner in the developer decreases as the development proceeds, because the toner is consumed by the development.
Thus, when the two-component developer is used, a developing apparatus should have a system comprising a means for detecting the concentration of toner and means for replenishing toner in response to a signal provided by the detecting means, thereby maintaining the toner concentration of the developer at a predetermined level (3-10%).
The detection of the toner concentration is generally performed by utilizing the phenomenon that the permeability of the developer varies depending on the toner concentration. For instance, a Hall element was proposed which is placed in a magnetic field of permanent magnet members to detect magnetic fluxes leaked from the developer (see Japanese Patent Laid-Open No. 51-117047). The Hall element shows high detection accuracy, but it is widely affected by temperature variations so that its output is not always reliable.
Thus, what is more widely utilized is a detection device comprising a coil placed in contact with the developer such that a part of its magnetic circuit is constituted by the developer, whereby the concentration of toner is detected as the variation of inductance of the detection coil (see, for instance, Japanese Patent Laid-Open Nos. 53-49437 and 54-159233).
In general, a developing apparatus for a two-component developer is equipped with a scrape adjacent to a non-magnetic sleeve of a magnetic roll to remove the developer remaining on the surface of the non-magnetic sleeve after developing operations so that the developer once used may be mixed with newly added toner. Proposals have been made to provide a guide member adjacent to the scraper to collect part of the developer falling from the nonmagnetic sleeve as a sample, and to carry out the detection of the toner concentration without being affected by the magnetic field of a permanent magnet in the magnetic roll (see, for instance, Japanese Patent Laid-Open Nos. 53-126944 and 54-76165). Further proposals have been made to provide detection coil above a scraper so that the detection coil can detect the toner concentration of the developer flowing on the scraper (see, for instance, Japanese Patent Laid-Open No. 59-164575). However, in this case, the detection surface of the coil is not in stable contact with the developer, resulting in variations of a developer density. Eventually, this fails to provide an accurate value of a toner concentration.